Transit is the second book in the Riverside series by Ben Aaronovitch, following the adventures of Detective Inspector Peter Grant as he navigates the world of supernatural policing in London. The novel opens with Grant investigating a seemingly routine overdose case, which quickly spirals into something much more sinister.

Grant discovers that the victim, a young woman named Simone Fitzwilliam, was addicted to a drug called "Transit," which not only causes hallucinations but can also transport the user to another dimension. As he delves deeper into the case, Grant realizes that Simone was not the only one using Transit, and that someone is manufacturing and distributing the drug on a large scale.

With the help of his colleague and former mentor, Detective Chief Inspector Thomas Nightingale, Grant begins to piece together the mystery behind Transit. They are soon led to an underground club called the "White Lodge," where he meets the charismatic and enigmatic Baroness Anglemark, who claims to have the answers they seek.

As Grant and Nightingale investigate further, they discover that the supernatural world is far more complex and dangerous than they ever imagined. They encounter all kinds of magical creatures, from revenants to unicorns, and must use their wits and their knowledge of magic to stay alive.

Along the way, Grant meets another supernatural creature, a river goddess named Beverly Brook, with whom he becomes romantically involved. However, their relationship is complicated by the fact that Grant is human and Brook is not and by the fact that Nightingale disapproves of their budding romance.

As the investigation progresses, Grant and Nightingale come to suspect that the true mastermind behind the Transit operation is one of Nightingale's old enemies, the Faceless Man, a powerful and dangerous magician who has a long-standing grudge against Nightingale.

In the thrilling climax of the book, Grant and Nightingale confront the Faceless Man and his army of magical creatures in an epic battle that will determine the fate of London and the magical world as a whole.

Overall, Transit is a thrilling and intricate urban fantasy novel that will keep readers on the edge of their seats. With its engaging characters, intricate plot, and well-crafted world-building, it is a must-read for fans of the genre.